Log photodiode

ABSTRACT

A light sensitive diode comprises an anode including a metal mesh and a wire probe disposed on the inner surface of a faceplate. A photocathode is disposed on a metal plate spaced opposite the anode and faceplate with the wire probe extending from the mesh close to the photocathode. Both anode and cathode have low resistances to permit application of maximum voltage across the gap. The device provides a non-saturating response with output signal current at the anode substantially proportional to the logarithm of input light radiation. Space charge limits, which normally occur at high light levels, are avoided in the area at the tip of the anode wire.

United States Patent 191 Appl. No.: 282,644

Eberhardt Mar. 19, 1974 LOG PHOTODIODE Primary ExaminerArchie R.Borchelt Assistant ExaminerD. C. Nelms t d H. Ebe h dt, W [75] Inven orE ward r at ayne Ind Attorney, Agent, or Firm-John T. Ol-lalloran; Me-[73] Assignee: International Telephone and mi J, L b rdi, J EdwardGoldberg Telegraph Corporation, Nutley, NJ. 1

[22] Filed: Aug. 21, 1972 [5 7] ABSTRACT A light sensitive diodecomprises an anode including a metal mesh and a wire probe disposed onthe inner surface of a faceplate. A photocathode is disposed on a metalplate spaced opposite the anode and faceplate with the wireprobeextending from the mesh close to the photocathode. Both anode andcathode have low resistancesto permit application of maximum voltageacross the gap. The device provides a non-saturating response withoutput signal current at the anode substantially proportional to thelogarithm of input light radiation. Space charge limits, which normallyoccur at high light levels, are avoided in the area at the tip of theanode wire.

6 Claims, 2 Drawing Figures [56] References Cited UNITED STATES PATENTS2,692,948 10/1954 Lion 250/211 2,034,586 3/1936 Long... 250/2113,641,352 2/1972 Fisher..... 250/213 VT 2,646,533 7/1953 Carne 313/973,688,145 -8/l972 Coles 313/99 LOG PHOTODIODE BACKGROUND OF THEINVENTION This invention relates to light sensitive diodes andparticularly to a photodiode which has a non-saturating or substantiallylogarithmic response at high light levels.

DESCRIPTION OF THE PRIOR ART The standard photodiodes used in the pastincorporated a planar photocathode on the inner face of a transparentfaceplate and a parallel anode spaced therefrom at a predetermineddistance at the opposite end of an evacuated envelope. Fixed potentialsare applied across the electrodes. For an increasing light input to thephotocathode, the output current at the anode increases proportionatelyuntil a space charge accumulates between the electrodes which causessaturation and flattening of the response curve. In addition, the outputsignal current is generally connected to an amplifier which alsosaturates at high signal levels.

An attempt to avoid this condition included the use of a conical anodehaving the tip closely spaced from the photocathode on the input glassfaceplate of the diode. ln this case, as light input increases, spacecharge saturation builds up at the larger gap outer peripheral areas ofthe full cathode diameter and progressively reduces the effectivesensitive area. However, due to the high field and smallspacing at thetip of the cone, the space charge limits are theoretically not reachedat that area. This configuration was subject to the problem of having afinite photocathode resistance which caused a reduced voltage to beapplied across the gap so that high currents at the tip brought thecathode'to the anode potential and resulted in undesired space chargesaturation at the tip. Another variation to eliminate the problem ofphotocathode resistance has also been suggested. This arrangementincludes a reversed diode having a conical photocathode on a metal basespaced from a planar metal mesh anode on the input faceplate. Both theanode mesh and photocathode layer thus have low resistances. Input lightflux penetrates the openings of the anode mesh to strike thephotocathode which emits electrons for collection by the anode. Theconical photocathode coating in thiscase however is somewhat difficultto apply.

SUMMARY OF THE INVENTION It is therefore the primary object of thepresent invention to provide an improved simplified photodiode which hasa non-saturating or logarithmic response at high light levels.

This is achieved by a novel electron tube configuration including ananode having a metal mesh positioned on the inner surface of an inputfaceplate and a wire probe extending'therefrom. A planar photocathode isdisposed on a conductive metal base plate spaced opposite the anode andfaceplate with the tip of the anode wire positioned in close proximityto the photocathode layer. At high input light levels, space chargesaturation occurs in the wide gap peripheral area but does not occur atthe tip which continues conducting over an extended range of lightradiation andcorresponding output signal current with a substantiallylogarithmic response. Low resistances of the photocathode and anodepermit application of maximum voltage across the gap and the planarcathode can be processed in a simplified manner. The two anode portionscan also be operated in parallel with combined current or used inseparate measuring or amplifier circuits to cover a wider dynamic range.Other objects and advantages will become apparent from the followingdescription taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a cross-section of anon-saturating photodiode configuration according to the presentinvention; and

FIG. 2 is a curve showing the improved output current response withinput light radiation.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, a source oflight 10 is directed toward a transparent faceplate 12, preferably ofglass, at one end of an evacuated tubular envelope 14. An anode 16including a planar metal wire mesh 18 and a separate wire probe 20 aredisposed on the inner surface of the faceplate. The mesh 18 extendslaterally within the tube across the faceplate and is of sufficienttransparency to permit light to pass through the openings of the meshinto the tube toward the photocathode with negligible interference. Theanode probe 20 extends longitudinally from the faceplate toward aphotosensitive electron emissive coating or layer 22 on a flat metalsubstrate or base plate 24 at the opposite end of the tube. Suitablepotentials, in the order of to 500 volts from a source of direct voltage26, are applied across the tube between the anode and cathode. The twoanode portions 18 and 20 have separate connections passing through thetube wall and faceplate respectively and are preferably connectedexternally in parallel circuits to the positive terminal of source 26.The photocathode layer 22 on base 24 is then connected to the negativeterminal of source 26.

The spacing between the tip of the anode wire 20 and photocathode layer22 is in the order of fractions of a millimeter, while that between theanode mesh 18 and layer 22 is in the order of centimeters. The probe may7 typically be about 1 centimeter in length and a fraction of amillimeter in diameter, while the mesh openings may also be in thefraction of a millimeter range. The anode and photocathode may be a fewcentimeters in diameter. Use of a metal substrate for the photocathodeminimizes resistance and avoids IR losses to permit application oflarger voltages across the electrodes. In

addition, the planar cathode provides a simplified structure for usewith standard coating processes.

As light flux entering the faceplate increases, the emission ofelectrons by the photosensitive cathode layer increases proportionatelyin a substantially logarithmic manner, as does output signal current inthe anode which collects the electrons. The generally known Langmuirrelation applies wherein the maximum current density is proportional tothe three halves power of the voltage applied and inversely proportionalto the square of the distance between electrodes, 1,; z (V3 2/d Thus aslight and electron emission increases, in the large gap peripheral areas28 between the mesh 18 and photocathode 22, a space charge of electronsbuilds up which limits output current and causes a saturating responsein those areas. However, due to the relatively high voltage across thesmall gap between the anode probe and photocathode layer with minimumresistance losses, space charge limits are not reached even atrelatively high light levels and conduction continues in anon-saturating manner in the small central area. The configuration alsopermits operation at much smaller voltages and output signal currents,in the order of milliamps rather than amps, as previously, and thus doesnot saturate the external amplifier circuits 30, 32,

34. Non-saturating action is extended in the order of 10 ten times theusual operating range.

FIG. 2 shows an example of response curves of input light radiation inwatts on a log scale versus output current in milliamps for a normalphotodetector, curve 36, and the improvement provided by the presenttube, curve 38. With 100 volts applied between the anode andphotocathode of a normal tube, having a photocathode sensitivity of lma/watt, a substantially linear response occurs until about watts inputradiation and 100 ma output current. At this point, the tube suddenlysaturates and further light produces no additional current. In thepresent case, with further input light applied of 100 watts, currentconduction continues in a non-linear relation to provide an outputcurrent of 200 ma, while at 1000 watts there is an output of 300 ma,without saturation.

An additional advantage of the split anode configuration, having aseparate mesh 18 and wire probe 20, is that the two portions can beconnected in parallel through individual amplifier circuits 30, 32 if sodesired, or combined in series with a common amplifier circuit 34. Theparallel operation, using two separate current measuring circuits withdifferent currents, may be advantageous in avoiding saturation of theindividual amplifiers since two amplifiers can cover a wider dynamicrange than a single one.

The present invention thus provides a novel improved photodiodeconstruction which can operate in a non-saturating manner atsubstantially higher light levels. While only a single embodiment hasbeen described and illustrated, it is apparent that other variations maybe made in the particular design and configuration without departingfrom the scope of the invention as set forth in the appended claims.

What is claimed is:

1. A light sensitive diode comprising:

an evacuated envelope,

a light transparent faceplate at one end of said envelope,

a metal base positioned within said envelope at the other end,

an anode disposed at the inner surface of said faceplate, said anodeincluding a metal mesh extending laterally across said envelopesubstantially parallel to said faceplate and a wire probe extendingaxially from said faceplate toward said other end,

a photocathode layer disposed on said metal base,

the extending end of said wire probe being positioned in close proximityto said photocathode at a predetermined spacing sufficient to avoidspace charge limits therebetween, and

a source of direct voltage applying a potential between said anode andphotocathode layer.

2. The device of claim 1 including means connecting said wire probe andmetal mesh to one terminal of said direct voltage source, the otherterminal being connected to said metal base and photocathode.

3. The device of claim 2 wherein said anode mesh and photocathode layerare disposed respectively on said faceplate and base on opposite planarparallel surfaces within said envelope and the other end of said wireprobe extends externally through said faceplate.

4. The device of claim 3 including first circuit means connected betweensaid one terminal and said wire probe and second circuit means connectedbetween said one terminal and said metal mesh.

5. The device of claim 3 wherein said potential and spacing between saidwire probe and photocathode provide continued current conductiontherebetween 'with a substantially non-saturating logarithmic retenththat between said anode mesh and photocathode. i 8

1. A light sensitive diode comprising: an evacuated envelope, a lighttransparent faceplate at one end of said envelope, a metal basepositioned within said envelope at the other end, an anode disposed atthe inner surface of said faceplate, said anode including a metal meshextending laterally across said envelope substantially parallel to saidfaceplate and a wire probe extending axially from said faceplate towardsaid other end, a photocathode layer disposed on said metal base, theextending end of said wire probe being positioned in close proximity tosaid photocathode at a predetermined spacing sufficient to avoid spacecharge limits therebetween, and a source of direct voltage applying apotential between said anode and photocathode layer.
 2. The device ofclaim 1 including means connecting said wire probe and metal mesh to oneterminal of said direct voltage source, the other terminal beingconnected to said metal base and photocathode.
 3. The device of claim 2wherein said anode mesh and photocathode layer are disposed respectivelyon said faceplate and base on opposite planar parallel surfaces withinsaid envelope and the other end of said wire probe extends externallythrough said faceplate.
 4. The device of claim 3 including first circuitmeans connected between said one terminal and said wire probe and secondcircuit means connected between said one terminal and said metal mesh.5. The device of claim 3 wherein said potential and spacing between saidwire probe and photocathode provide continued current conductiontherebetween with a substantially non-saturating logarithmic response ofoutput current over an extended range of light radiation input afterspace charge build up between peripheral anode and photocathode areasupon the occurrence of a predetermined light input.
 6. The device ofclaIm 5 wherein the spacing between said wire probe and photocathode isin the order of one tenth that between said anode mesh and photocathode.